With the global population surging and consequent increased demand for animal protein, the consumption of antibiotics in veterinary and human health has significantly escalated [1]. It is estimated that the worldwide antibiotic consumption reaches approximately 100 million kilograms annually, with over 25% attributed to China [2]. Sulfamethazine (SMZ), a commonly used antibiotic, is extensively applied in the medical field and animal husbandry for its antibacterial properties and as growth promoters [3,4]. The excessive or prolonged misuse of SMZ in livestock can lead to residues in animal-derived foods, and potentially causing toxic reactions in humans [5]. European Union, United States of America, and China have established a maximum residue limit for SMZ in milk as 25 µg/L [6]. Furthermore, it is reported that about 30%–90% of SMZ may be discharged into the aquatic environment through domestic sewage, medical wastewater, aquaculture wastewater [7]. And, this cause could result in SMZ concentrations in water ranging from 0.21 µg/L to 3.15 µg/L [8]. The direct or indirect intake of SMZ poses potential risks to human health and ecological security [9]. It has been reported that at least 700,000 people die each year from antibiotic-related events globally, and a number that may rise to 10 million by 2050 [10]. Therefore, it is of great importance to develop an efficient and convenient sensing technique for SMZ detection at low concentration level.
Over the past decades, multiple methods have been developed for SMZ detection, including high-performance liquid chromatography (HPLC) [11,12], gas chromatography-mass spectrometry (GCMS–) [13], colorimetry [14,15], fluorometry [16], enzyme-linked immunosorbent assay (ELISA) [17] and surface enhanced Raman spectroscopy (SERS) [18]. HPLC and GC–MS can accurately determine the concentration of SMZ in the range of 10–5000 ng/mL in 40 min ~ 1.5 h with a detection limit (LOD) of 1–2.8 ng/mL [11-13]. However, the high-cost, complex analyses and lengthy procedures limit their application in rapid environmental hazards detection [19]. While, sensing detection technologies based on colorimetry, fluorometry and ELISA can effectively reduce analytical time to 30–60 min with convenient operation steps [14-17]. However, these methods show disadvantages such as poor stability, aptamer dependence, or the complexity of functional material preparation [20,21]. SERS is an effective technology for low-concentration SMZ detection with high sensitivity (0.1 pg/mL), rapid on-site monitoring (less than 15 min), and selective recognition [18]. Despite these advantages, the preparation of clean substrates with strong stability, well reproducibility and high enhancement still remains a challenge for SMZ detection via SERS technique [22]. Therefore, there is an urgent need to develop a convenient, efficient and rapid detection technology for SMZ detection.
In recent years, electrochemical sensors have been used in many fields such as food safety testing, environmental monitoring and biological diagnosis [23]. It is a technology that utilizes the redox reactions between sensitive materials and target pollutants to detect contaminants, which has the advantages of high sensitivity, simple operation and easy miniaturization [24,25]. Research indicates that the amino groups in SMZ molecules undergo anodic oxidation at the electrode-electrolyte interface to generate electrical signals [26]. Glassy carbon, carbon paste and boron-doped diamond electrodes have all been used to detect SMZ in aquatic environment [27,28], but the single electrode is difficult to fulfill the detection accuracy requirements [29]. Gold nanoparticles (AuNPs) have the advantages of low resistivity (2.05 × 108 Ω m) and easy surface modification [30]. The electrical signal for contaminants detection can be significantly amplified using AuNPs-modified electrodes [31]. However, AuNPs have problems of easy oxidation and spontaneous agglomeration [32]. para-Sulfonated calix[4]arene (pSC4), a class of water-soluble calixarenes with rigid and open cavities, has a considerable binding ability towards guest molecules [33]. pSC4 can bind to AuNPs through its sulfonic groups to form stable Au-S bond, and inhibiting the aggregation of AuNPs [34]. Moreover, the macrocyclic cavity in pSC4 has been shown to selectively bind to the -NH2 groups through host-guest interactions [35]. However, the signal intensity of the pSC4 modified electrode has always been problematic. Therefore, the development of pSC4 functionalized AuNPs (pSC4-AuNPs) modified electrode may provide a new approach for the detection of low concentration SMZ in aquatic environment or food residues.
In this work, by binding AuNPs with pSC4 to enhance the conductivity of pSC4, and then modifying pSC4-AuNPs on glassy carbon electrode (GCE) to construct a sensitive and convenient electrochemical sensor for SMZ detection (Fig. S1 in Supporting information). UV–vis spectra and atomic force microscopy (AFM) analysis was applied to have insight into the interaction mechanism between pSC4-AuNPs and SMZ. Electrochemical characteristic analysis was performed to verify the feasibility of electrochemical SMZ detection. The specificity of the constructed sensor for target SMZ was proofed by the interference and selectivity experiments. And the application of the sensor to detect SMZ in tap water confirmed its feasibility in actual water environment. Capitalizing on the electrical properties of gold nanoparticles and the chemical attributes of supramolecular, this study offers a novel pathway for the sensitive and convenient electrochemical detection of small molecular environmental contaminants.
pSC4 (>94%), hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, 49.0% Au), hydrogen phosphate (Na2HPO4, 99%), potassium ferricyanide (K3[Fe(CN)6], 99%), potassium ferrocyanide trihydrate (K4[Fe(CN)6]·3H2O, 99%) and sulfuric acid (H2SO4, 98.3%) were bought from Sigma Aldrich (Shanghai) Trading Co., Ltd. China. Potassium chloride (KCl, 99.5%), anhydrous ethanol (≥ 99.8%) and sodium borohydride (NaBH4, 98.8%) were purchased from Sigma-Aldrich, Inc. (St. Louis, USA). SMZ (99%), tetracycline (TC, 98%), chloramphenicol (CHL, 98%), dimetridazole (DMZ, 98%), enrofloxacin (ENR, 99.5%) were synthesized by Sangon Biotech Co., Ltd. (Shanghai, China). All solutions were prepared using deionized water (DW, 18.2 MΩ cm) purified by Millipore Milli-Q purification system (Branstead, USA).
All reagents were weighed by an electronic analytical balance an accuracy of 0.01 mg (XS105, Mettler Toledo, Switzerland). UV–vis spectra were obtained via a UV-2450 spectrophotometer (Shimadzu, Japan). Nano particle size and zeta potential were carried out with a Zetasizer 3000HS system (Malvern, UK). AFM were performed utilizing an Agilent 5500 instrument (Santa Clara, USA). cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested by Autolab PGSTAT128 N electrochemical workstation (Eco Chemie B.V., the Netherlands). GCE, saturated calomel electrode, platinum wire and manual pipette were purchased from Aida Hengsheng Technology Development Co., Ltd. (Tianjin, China).
The pSC4-AuNPs was synthesized by sodium borohydride reduction method [36]. pSC4 solution (2.8 mL, 10.0 mmol/L) and HAuCl4 solution (2.8 mL, 26.3 mmol/L) were added to 126 mL of DW in turn at room temperature and stirred for 20 min. Subsequently, 2.8 mL of freshly prepared NaBH4 solution (10.0 mmol/L) was rapidly added to the mixture, and the resulted solution was vigorously stirred for 2.0 h The color of the solutions gradually changed from bright yellow to deep red. Then, the synthetic pSC4-AuNPs were washed and centrifuged for three times to remove free pSC4. Finally, the purified pSC4-AuNPs colloids were stored at 4 ℃ in amber laboratory bottles. All experiments were conducted in dark environment.
In general, the directional modification of GCE was accomplished by the drop-coating method [37]. Firstly, 5.0 mg of SMZ was dissolved in 250.0 mL of dilute H2SO4 (0.25 mol/L) and diluted step by step to obtain SMZ solutions with concentrations of 1.0, 10.0, 1.0 × 102, 1.0 × 103, and 1.0 × 104 ng/mL. Secondly, 10 µL of SMZ solutions with different concentrations were dropped into 90 µL of pSC4-AuNPs suspension at room temperature. After sufficient reaction, the color of the mixture was changed from burgundy to gray purple, resulting in the formation of the pSC4-AuNPs/SMZ suspension. Then, the GCE (3.0 mm diameter) was washed several times with DW until clean and dried by nitrogen. Next, the cleaned GEC was meticulously polished with 0.3 µm and 0.05 µm alumina paste to achieve a smooth surface. Following that, the polished GCE was sequentially fully sonicated in DW and ethanol to remove residual alumina powder. Afterward, the GCE surface was dried with nitrogen. Finally, a volume of 20 µL of pSC4-AuNPs or pSC4-AuNPs/SMZ with different concentrations was drop-coated on the surfaces of well-treated different GCE, respectively, and dried at 60 ℃ for 40 min. Complete all the above steps, the modified GCE was ready for usage in the electrochemical sensing application (Fig. S1).
The electrochemical measurements by using CV and EIS for SMZ detection were conducted in an Autolab PGSTAT128 N electrochemical workstation with a three-electrode system at room temperature. Saturated calomel electrode and platinum wire were utilized as reference electrode and counter electrode, respectively. CV signal was measured with a potential voltage range from −0.8 V to 0.8 V at 100 mV/s scan rate. EIS measurements were collected in 5.0 mmol/L [Fe(CN)6]3-/4- electrolyte solution containing 0.1 mol/L KCl (pH 7.4), and the frequency range was from 0.1 Hz to 100 MHz. All experiments were repeated three times. And the corresponding standard deviations were calculated according to the square root of the arithmetic mean of the square of the mean difference (Text S1 in Supporting information), which was expressed in the form of error bars in this study.
The recognition between pSC4-AuNPs and SMZ was analyzed by UV–vis spectroscopy. As shown in Fig. 1A, the pSC4-AuNPs showed a significant absorption peak at 527 nm, which is the typical UV–vis spectra of AuNPs [38]. After adding dilute H2SO4, the peak intensity at 527 nm was slightly weakened. Upon further introducing SMZ into the reaction system, the peak intensity at 527 nm was significantly reduced and a new absorption peak at 665 nm appeared. This result may be attributed to the aggregation of AuNPs [14]. Recent research has demonstrated that the host-guest recognition between the -NH2 in SMZ and the macrocyclic cavity in pSC4 could induce the aggregation of pSC4-AuNPs [39]. Furthermore, laser nanoparticle size and zeta potential were the crucial parameters for characterizing the interaction between pSC4-AuNPs and SMZ. In the reaction system without addition of SMZ, the zeta potential and size of the pSC4-AuNPs was −26.8 mV and 39.6 nm (Fig. 1B), respectively, suggesting the well dispersion of the material [38]. While, as the increasing concentration of SMZ, the size and zeta potential of pSC4-AuNPs/SMZ was gradually decreased and increased, respectively. The results imply that the attraction between pSC4-AuNPs is enhanced, leading to a propensity for aggregation [40]. Thus, the aggregation phenomenon triggered by the host-guest interaction between pSC4-AuNPs and SMZ was further confirmed.
To further verify the recognition effect of pSC4-AuNPs on SMZ, AFM was used to identify the morphology changes before and after the interaction between pSC4-AuNPs and SMZ [41]. As illustrated Fig. 1C, the roughness of pSC4-AuNPs modified chip surface was only 7.2 nm. After the incorporation of pSC4-AuNPs/SMZ, the roughness significantly increased to 15.5 nm and showed an obviously irregularity (Fig. 1D). This may be due to the aggregation of pSC4-AuNPs induced by SMZ, and the formation of stable Au-S bond between the -SO3- in pSC4 and the AFM probe [42], and further leading to a rougher surface on the pSC4-AuNPs/SMZ modified chip than the pSC4-AuNPs one. These results confirmed that the synthesized pSC4-AuNPs is suitable for recognition of SMZ.
To evaluate the feasibility of the designed electrochemical sensor based on pSC4-AuNPs for SMZ detection, the stepwise modification process was monitored by CV and EIS [43]. Compared to the unmodified GCE, the current and Rct values of the pSC4-AuNPs modified GCE are significantly enhanced and decreased (Fig. 2). These results may be attributed to the excellent electrical conductivity exhibited by pSC4-AuNPs as electrode modification material, which significantly enhanced the electron transfer in the detection system [44]. However, the pSC4-AuNPs/SMZ modified GCE showed the weakest peak current of anodic and cathodic. EIS values showed a completely opposite trend to CV. These results reveal the reduction of electron transfer in the detection system. The inhibition of electron transfer on the electrode surface may be due to the aggregation of pSC4-AuNPs/SMZ that reduced the binding sites on the modified material surface [45]. In addition, Fig. 2A also shows the redox effect potential difference (ΔE) of different electrodes. The ΔE value of pSC4-AuNPs modified GCE was higher and lower than GCE and pSC4-AuNPs modified GCE, indicating the presence of SMZ impeded the charge transfer on the electrode surface. The above results suggested that pSC4-AuNPs/SMZ modified on the electrode interface could enhance the electrochemical response signal, which confirmed feasibility for the effective SMZ detection via the constructed sensor.
The sensing conditions, such as the modified volume of pSC4-AuNPs and SMZ, pH values of pSC4-AuNPs and the modification time were optimized by the EIS signals with 100 ng/mL of SMZ. As shown in Fig. S2A (Supporting information), the impedance gradually increased and reached saturation at 90 µL with the increasing of the pSC4-AuNPs amount, indicating that more amount of SMZ was attached through the specific recognition of pSC4. It also can be observed that the Rct value increased over time and becomes stable at the SMZ volume of 10 µL (Fig. S2B in Supporting information). These phenomena were reported in the previous study, and attributed to the reduction of target analyte or recognition site in the detection system [46]. Here, the optimal modified volume of pSC4-AuNPs and SMZ were 90 and 10 µL, respectively. Fig. S2C (Supporting information) exhibits the effect of pH value on SMZ detection in the electrochemical sensing system. As the pH values increased from 4.0 to 8.0, impedance shows a trend of increasing firstly and then decreasing, and the highest Rct value appeared at the pH value of 7.0. It has been reported that the protonation or deprotonation mechanism of SMZ may lead to a negative shift of the oxidation peak potential in the electrochemical sensing system, thereby inhibiting the oxidation of SMZ [27]. Therefore, the optimal pH value was found to be 7.0. Moreover, the modification time of pSC4-AuNPs/SMZ plays an important role for SMZ detection [47]. As shown in Fig. S2D (Supporting information), the Rct values increased and stabled at 60 min with the modification time increased. This result may be due to the formation of stable pSC4-AuNPs/SMZ modified layer on the GCE surface, which enhanced the electrochemical signal. With the passage of time, the physical adsorption and chemical bonding effects gradually reached equilibrium during the formation of the modified layer, resulting in the stabilization of the signal [48]. Thus, the 60 min was selected as the optimal modification time.
The sensing sensitivity of the developed electrochemical sensor was evaluated by measuring EIS response to SMZ with different concentrations (1.0, 10.0, 1.0 × 102, 1.0 × 103 and 1.0 × 104 ng/mL) in 5.0 mmol/L [Fe(CN)6]3-/4- electrolyte solution containing 0.1 mol/L KCl (pH 7.4). Under optimal conditions, the recognition strategy was performed via the host-guest interaction between SMZ and pSC4, and the signal amplification strategy was achieved by the aggregation of pSC4-AuNPs. The EIS signals intensified continuously with the increasing SMZ concentration (Fig. 3A). Meanwhile, the Rct values of the detection system with different SMZ concentrations are shown in Fig. 3B. A significant correlation between Rct values and the logarithmic value of the SMZ concentration ranging from 1.0 ng/mL to 1.0 × 104 ng/mL was observed (Eq. 1). According to the 3′ s blank criterion, the LOD was calculated to be 0.0038 ng/mL (Eq. 2). The linear equation regression and the calculation equation were presented as follows:
|
|
(1) |
|
|
(2) |
where Rct is the EIS signal of pSC4-AuNPs/SMZ modified GCE, CSMZ is the SMZ concentration, σ is the standard deviation of blank signal (n = 3), S is the slope of the linear regression equation.
Compared with previously reported detection methods in Table S1 (Supporting information), the electrochemical sensor constructed in this work showed a wide linearity range and a low LOD for SMZ detection. Notably, the linearity rang and LOD values were about 2105 and 10 times than the colorimetric method based on pSC4-AuNPs, respectively. Such a low LOD may be attributed to (1) the dramatic signal enhancement ability induced by AuNPs aggregate, and (2) the host-guest recognition between pSC4-AuNPs and SMZ [48]. Therefore, the sensor constructed in this study had a new concept for SMZ detection performance for simple and sensitive detection of SMZ in water.
Specificity is a crucial metric for assessing the performance of sensors [49]. To evaluate selectivity capture capability for SMZ detection by the electrochemical sensor, multi-antibiotics like CHL, TC, ENR, DMZ with concentration (10 ng/mL) of 100 times than SMZ (100 ng/mL) are used as interfering substances in the control experiments. As shown in Fig. 4A, the average Rct values for CHL, TC, ENR and DMZ detection are 29.7, 40.8, 41.1 and 78.9 Ω, respectively, which was obviously lower than that of SMZ (625.2 Ω). Compared with the four interfering substances, the macrocyclic cavity in pSC4 molecule can recognize the unique -NH2 group in target SMZ molecule through the host-guest interaction, including the π-π stacking, hydrophobic interaction, electrostatic interaction and p-π conjugation [6]. These outcomes demonstrate the constructed sensor the excellent selectivity for SMZ detection.
To evaluate the stability of the proposed electrochemical sensing method, a long-term stability experiment was conducted by measuring the EIS signals of pSC4-AuNPs/SMZ modified GCE three days interval. After 21 days of storage, there was no significant change in impedance values with a relative standard deviation (RSD) was 1.1% that less than 5.0% (Fig. 4B). This result indicates that outstanding stability of the constructed sensor.
To investigate the practical and reliability of the proposed electrochemical sensor, the spiked recovery experiments for the detection of SMZ with different concentrations (0.1, 1.0 and 10.0 ng/mL) were carried out in the tap water. As illustrated in Table S2 (Supporting information), the recoveries ranged from 91.1% to 97.0%, and the RSDs were in the range of 1.5% to 3.5% (less than 5%). This result showed a certain accuracy for SMZ detection in tap water, indicating that the constructed electrochemical sensor can be used for SMZ detection real water environment.
The development of innovative pathway for electrochemical sensing detection of SMZ is highly desirable. pSC4-AuNPs was fabricated for sensitive electrochemical material with significantly potential in the detection of small molecule. In this study, pSC4-AuNPs exhibited a specificity recognition and excellent electronic transmission for SMZ detection. The SMZ mediated aggregation of AuNPs through the host-guest recognition with pSC4 significantly amplificated the electrochemical signals for SMZ assay. The quantitative SMZ detection by the rapid, sensitive and stable electrochemical sensor with the LOD of 0.0038 ng/mL. The recoveries ranged from 91.1% to 97.0% for the detection of SMZ in tap water. The intrinsic mechanism of the specific host-guest interactions provides a new idea for the future design of ultrasensitive detection platforms for SMZ in the environmental monitoring.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Zhijuan Niu: Writing – original draft, Visualization, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation. Peizhe Sun: Writing – review & editing, Validation, Supervision, Methodology, Data curation. Kwangnak Koh: Writing – review & editing, Supervision, Data curation, Conceptualization. Changping Li: Writing – review & editing, Validation, Supervision, Resources.
This study was financially supported by National Natural Science Foundation of China (No. 42301095).
Supplementary material associated with this article can be found, in the online version, at doi:
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Figure 1 (A) UV–vis absorption spectra of pSC4-AuNPs, pSC4-AuNPs/H2SO4 and pSC4-AuNPs/SMZ. (B) Zeta potential and size characteristics of pSC4-AuNPs interaction with SMZ. AFM images and line profiles (inset) of (C) pSC4-AuNPs and (D) pSC4-AuNPs modified gold surface.
Figure 2 (A) CV curves and (B) EIS spectra of GCE, pSC4-AuNPs/GCE and pSC4-AuNPs/SMZ/GCE.
Figure 3 (A) EIS spectra of pSC4-AuNPs/SMZ modified GCE, inset is the color of pSC4-AuNPs solution before (left) and after (right) adding SMZ with different concentrations. (B) Linear range of SMZ detection.
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